The present invention relates to a process for production of sulfur from SO.sub.2 -containing gas in which a SO.sub.2 reduction reaction using coal, coke or the like and a Claus reaction of H.sub.2 S and COS with unreacted SO.sub.2 generated during said SO.sub.2 reduction reaction are utilized.
As a process for production of an elemental sulfur from SO.sub.2 -containing gas from the activated carbon regenerator or the like of a dry desulfurization plant, there has been known a process for producing sulfur using carbonaceous particles of coal, coke or the like as a reducing agent, at a temperature of 621.degree.-843.degree. C. by a reaction such as SO.sub.2 +C.fwdarw.S+CO.sub.2. However, this process is defective in that, due to the co-existence of steam in SO.sub.2 -containing gas, side reactions such as a water gas reaction and the like unavoidably take place in the process of reduction of SO.sub.2 to thereby produce H.sub.2 S, COS and the like, whereby a high sulfur productivity can not be achieved. In view of this, public attention has recently been attracted to a process using a SO.sub.2 reduction reactor and a Claus reactor in series, said process comprising, producing sulfur through said reduction reaction in the reduction reactor of the first stage, supplying H.sub.2 S, COS and unreacted SO.sub.2 by-produced in the process of this SO.sub.2 reduction reaction to the Claus reactor of the second stage, and further producing sulfur through Claus reaction: 2H.sub.2 S+SO.sub.2 .fwdarw.3S+2H.sub.2 O.
The most serious problem with the process for recovery of sulfur using the SO.sub.2 reduction reactor in series and Claus reactor maintaining the molar ratio of H.sub.2 S/SO.sub.2 of the gas supplied to the Claus reactor of the second stage at a value optimum to the Claus reaction. Generally speaking, there is a tendency that the SO.sub.2 and H.sub.2 S concentrations in the gas coming from the SO.sub.2 reduction reactor depend upon the amount of oxygen (air) supplied additionally to the SO.sub.2 -containing gas, namely when the amount of oxygen (air) increases, the SO.sub.2 concentration lowers and inversely the H.sub.2 S concentration rises. Therefore, it is theoretically possible to maintain the molar ratio of H.sub.2 S/SO.sub.2 of the gas supplied to the Claus reactor at a value optimum to the Claus reaction by regulating the above mentioned amount of oxygen (air). However, there exists an extremely important problem of what should be selected as a controlled variable for regulating the amount of oxygen (air) supplied.
Japanese Laid Open Patent Application 167107/1980 teaches that the amount of air supplied should be controlled so that the SO.sub.2 reduction temperature may be controlled at a temperature of 850.degree.-950.degree. C. on the basis of the discovery that the molar ratio of H.sub.2 S/SO.sub.2 of gas coming from the SO.sub.2 reduction reactor can be maintained by carefully controlling the reaction temperature in the SO.sub.2 reduction reactor, concretely by reducing SO.sub.2 at a temperature of 850.degree.-950.degree. C. In other words, this process selects the reaction temperature in the SO.sub.2 reduction reactor as the controlled variable for regulating the amount of air supplied. However, it is not necessarily commendable to use the reaction temperature as the controlled variable. The reason is that since the temperature in the SO.sub.2 reduction reactor is affected by the potential heat of preheated SO.sub.2 -containing gas, the combustion heat of carbonaceous particles, the heat of reaction in the reactor, radiant heat from the reactor and the like, a very complicated temperature distribution is formed in the SO.sub.2 reduction reactor. In order to maintain a fixed reaction temperature, as a matter of course it is necessary to measure the temperature in the reactor. This temperature measurement involves the following problems. When the temperature in the reactor is measured by a thermometer directly installed in the reactor, the thermometer is corroded in short period of time by a high temperature atmosphere and so must be exchanged at relatively frequent intervals. This is very troublesome. On the other hand, when the temperature in the reactor is estimated by a thermometer installed in the side wall of the reactor, on the one hardly expected to obtain an estimated value of the reactor temperature with satisfactory accuracy. To sum up, the method taught by said laid open patent application comprising regulating the amount of air supplied so that the SO.sub.2 reduction reaction may take place at a temperature of 850.degree.-950.degree. C. by monitoring the temperature in the reactor is not practical.
Y. Seike, one of the present inventors, has proposed, in U.S. patent application Ser. No. 317,382 filed Nov. 2,1981, a process of regulating the amount of oxygen (air) to be supplied to the SO.sub.2 reduction reactor which comprises directly measuring the concentrations of H.sub.2 S, SO.sub.2 and if needed COS of the outlet gas of the Claus reactor and employing the measured values thereof as the controlled variables. The process of regulating the amount of oxygen (air) by employing the H.sub.2 S concentration and SO.sub.2 concentration of the outlet gas from the Claus reactor as the controlled variables is an improvement as compared with the process which employs the reaction temperature in the SO.sub.2 reduction reactor as the controlled variable, but is still defective in that expensive measuring instruments and tools are required for measuring not only the SO.sub.2 concentration but the H.sub.2 S concentration.
In this situation, the present inventors have discovered that when the SO.sub.2 -containing gas is relatively stable in SO.sub.2 concentration, for instance, the SO.sub.2 -containing gas comes from the regenerators of conventional dry desulfurization plants, and this gas is added with oxygen (air) and treated in the SO.sub.2 reduction reactor and then a Claus reactor disposed on the downstream side thereof, the molar ratio of H.sub.2 S/SO.sub.2 or (H.sub.2 S+COS)/SO.sub.2 of the outlet gas of the SO.sub.2 reduction reactor (these molar ratios are generically named the "Claus ratio" hereinafter) depends upon the SO.sub.2 conversion rate in the SO.sub.2 reduction reactor irrespective of the kinds of carbonaceous particles used in the SO.sub.2 reduction reactor. When the conversion rate is in the range of 75-85% the Claus ratio of the outlet gas of the SO.sub.2 reduction reactor becomes 2, and further the SO.sub.2 conversion rate depends upon the amount of oxygen (air) introduced into the SO.sub.2 reduction reactor. Still further, we have discovered that irrespective of the kinds of carbonaceous particles used in the SO.sub.2 reduction reactor, the Claus ratio of the outlet gas of Claus reactor has a definite fixed relationship with the SO.sub.2 concentration of the outlet gas of the Claus reactor which depends upon the amount of oxygen (air) introduced into the SO.sub.2 reduction reactor similarly.